1. Field of the Invention
The present invention relates to a radio transmitter and a radio transceiver for handling a modulated wave signal having an amplitude component, as well as a high frequency power amplifier used in the same.
2. Prior Art
In general, when a modulated wave signal having an amplitude component is amplified by a high frequency power amplifier, a signal up to a peak power needs to be amplified linearly. This requires a large DC power, and causes a waste of DC power by the amount of difference (back-off) of the peak power from the mean power.
For example, in a class A amplifier, its theoretical maximum efficiency is 50%. When the back-off is 7 dB, the mean power falls to as low as 20% of the peak power so that the efficiency falls to 10%.
In order to resolve such a problem, an EER (Envelope Elimination and Restoration) method known as the Kahn technique has been proposed in the prior art (for example, see U.S. Pat. No. 6,256,482B1(FIG. 6 at p. 3 of drawing sheets)).
FIG. 8 is a block diagram showing the outline of an EER method. In FIG. 8, a modulated wave inputted to an input terminal 40 is branched. In one branch, a detector 41 performs envelope detection on the modulated wave so as to generate an amplitude component. A DC converter 42 for receiving a supply voltage Vdd is composed of an amplifier for amplifying a DC component. This DC converter 42 performs DC conversion on the amplitude component described above. The DC converter 42 is implemented by a class S amplifier capable of operating at an efficiency of 95% or higher, such as a switching regulator. The output voltage of the DC converter 42 is provided as a supply voltage to a saturation amplifier 44.
In the other branch, the modulated wave is inputted to a limiter (amplitude limiting amplifier) 43, and thereby undergoes amplitude limitation. As a result, a modulated wave having solely phase information is obtained from the limiter 43. The modulated wave having phase information is inputted to an RF input terminal of the saturation amplifier 44, and thereby modulates the gate voltage of a field effect transistor or the like constituting the saturation amplifier 44. As a result, a modulated wave is outputted from an output terminal 45 provided in the saturation amplifier 44.
The saturation amplifier generically indicates an amplifier used in a state that a change in the output power is no longer caused by a change in the input power, or alternatively immediately below this state. Such an amplifier operates at the theoretical maximum efficiency for the operation class.
As described above, the EER method can employ a saturation amplifier having a high efficiency. This improves the efficiency in the high frequency power amplifier section.
Nevertheless, such a saturation amplifier shows nonlinear response to an amplitude component provided as a supply voltage, as well as to a phase component inputted as a modulated wave. This causes the necessity of a distortion compensation circuit for precisely canceling out the nonlinear response.